For conventional copying machines in which an electrostatic latent image formed on a photosensitive body is developed by a development apparatus and then transferred onto recording paper, many development techniques have been proposed which employ a two-component magnetic brush that brings developer into contact with the surface of the photosensitive body to make the electrostatic latent image visible. This method has become the mainstream because of its good picture quality and picture quality retaining capability although it has a drawback of making the toner density control complex and increasing the size of the apparatus.
As an increasing number of copying machines have begun introducing higher image quality and multicolor recording in recent years, multicolor image forming processes are being proposed in which two or more color-toner images are formed simultaneously on the photosensitive body and then transferred onto recording paper at a time. If the above-mentioned contact-type two-component magnetic brush development method is applied to the second or later development stage during the multicolor image formation, various technical problems arise, i.e. toner images already formed on the photosensitive body are destroyed, color-toners other than the selected one are mixed, or other color-toners enter into the development apparatuses.
To solve these problems, a number of so-called non-contact development techniques have been proposed to develop latent images without bringing the developer into contact with the surface of the photosensitive body.
Among such non-contact development methods is Japanese Patent Laid-Open No. 144452/1981, in which a thin layer of developer made up of toner and magnetic carrier (i.e. development magnetic brush) is formed on the development roll and, with the photosensitive body and the development magnetic brush kept out of contact, disturbing effects are produced in the developer layer by a magnetic, electrical or-mechanical means to develop the image.
This method allows the development stages located behind the first toner image in the multicolor image forming process to form quality images with no disturbance that would otherwise result from mechanical contact of the magnetic brush and with no mixing of colors.
As disclosed in Japanese Patent Laid-Open No. 176069/1985 in particular, magnetic poles are installed in the magnet roll at positions away from the point where the development sleeve is closest to the photosensitive body, and a horizontal magnetic field is applied to the developer layer which is also subjected to a vibrating electric field for development. This method assures formation of a thin, uniform layer of developer, which in turn enables quality images to be formed by development stages located behind the first toner image in the multicolor image forming process.
These development methods can not only be applied to the multicolor image forming process but also be used as monocolor recording development techniques. Since the magnetic brush is out of contact with the latent image on the photosensitive body, an input image can be expected to be reproduced precisely.
Although a thin and uniform developer layer can be obtained by the action of the horizontal magnetic field, the development method proposed by Japanese Patent Laid-Open No. 176069/1985, however, has a drawback of deteriorated development performance. That is, in the multicolor image forming process,to prevent a toner image already formed on the photosensitive body from being disturbed, the development performance for the current image being developed cannot be enhanced to a sufficient degree.
In an effort to resolve the above problem of deteriorated development performance in the non-contact development process, a variety of studies have been made on the development method in which the development magnetic poles are located away from the position where the sleeve is closest to the photosensitive body to apply a horizontal magnetic field to the developer layer during the development process.
Our studies have found that when a horizontal magnetic field is applied, the developer particles are magnetically aligned along magnetic lines of force forming concentrated, uniform thin layers of developer over the surface of the development sleeve. It is, however, found that only one or two top developer layers contribute to the development.
This is considered due to the fact that because the developer layers are densely packed along the lines of magnetic force, toner particles in the lower layers near the development sleeve cannot move up to the surface of the top layer.